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Your Mac's chip is powerful enough to handle the latest Sierra. This is probably some other issue and we need to figure it out. First we can Disk Utility and see if it works or not. Turn off your Mac by holding down Power button for 10 seconds. Then turn it back on and immediately hold down Command and R keys at same time for 20 seconds and this will boot your Mac to the Mac OS X Utilities screen. From there, click Disk Utility. Select your hard disk at left column (Macintosh HD or hard disk name). Click First Aid button and run it. Repair if asked.
Then restart from Apple menu. If that didn't work, then turn it off. Turn it back on and immediately hold down Command, Option, P and R keys at the same time for 5 seconds, then release the keys and immediately hold down Shift key for 20 seconds. This will boot into desktop running some repairs. Then restart normally.
Please let me know if that didn't work, so we can continue troubleshooting.
I don't have access to any of your credit card details to cancel myself. I'm only a tech in here. For that, you have to contact the customer support at***@******.*** to cancel it.